绘制图像时抛出Null Pointer Exception

Question

我正在尝试制作一个相当简单的游戏，有一个玩家类在尝试绘制时实例化异常。以下代码是播放器类。它被称为在主游戏类中创建玩家对象。

package space.invaders;

import java.awt.Graphics;
import java.awt.Image;
import java.awt.Rectangle;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.io.IOException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.JPanel;

/**
  *
* @author Your Name
* <Tristan, Teacher: Anthony , Last Edited: , Date Created, Program Description>
*/
public class player extends JPanel{
public static int px,py;   
public static int rx = 1, ry = 1, width =75, height = 50;
public Image player;
static Rectangle shot = new Rectangle(rx,ry,width,height);
static Graphics g;
String imagePath;
URL playerPath;



// draws the palyer    
public void paintPlayer(){
    playerPath = getClass().getResource("/graphics/player.png");
    try{
        player = ImageIO.read(playerPath);
    }catch(IOException io){
        System.out.print("The player picture didn't load properly");
    }
    g.drawImage(player, px, py, player.this);
}
// draws the shot
static void paintShot(){
    g.drawRect((int) shot.getX(), (int) shot.getY(),(int)shot.getWidth(),(int)shot.getHeight());
}
// moves the shot upwards, though might be unnececcary
static int moveShot (){
    for(ry = (int) shot.getY(); ry < 300; ry++){
        return ry;
    }
    return ry;
}

int setX(int x){
    return px;
}

int setY(int y){
    return py;
}

// I think this can be void since your * reminder to test that *
String setImage(String imagePath){        
    return imagePath;
}



// player movement control
public static class movement implements KeyListener {

    @Override
    public void keyTyped(KeyEvent e) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    @Override
    public void keyPressed(KeyEvent e) {
        if(e.getKeyCode() == KeyEvent.VK_RIGHT){
    rx += 5;

    }
    else if(e.getKeyCode() == KeyEvent.VK_LEFT){
    rx -= 5;

    }
    else if(e.getKeyCode() == KeyEvent.VK_UP){
    ry -= 5;

    }
    else if(e.getKeyCode() == KeyEvent.VK_DOWN){
    ry += 5;
    }
        else if(e.getKeyCode() == KeyEvent.VK_SHIFT){
            paintShot();
            moveShot();
        }
    }

    @Override
    public void keyReleased(KeyEvent e) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    }
}`

这是主要的游戏类，它正在调用和创建入侵者和玩家类（与玩家存在的入侵者问题相同）

package space.invaders;

import java.awt.Color;
import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JPanel;
import space.invaders.player.movement;

/**
 *
 * @author Your Name
 * <Tristan , Teacher: Anthony , Last Edited: , Date Created, Program Description>
 */
public class mainGame extends JFrame{
    public player p1 = new player();
    public mainGame(){        
        EventQueue.invokeLater(new Runnable(){
            @Override
            public void run() {
                createGUI();
                startGame();
                drawInvaders(1);

            }            
        });
    }
    void createGUI(){
        JFrame gameWindow = new JFrame("Space Invaders");
        JPanel observableWindow = new JPanel();
        gameWindow.add(observableWindow);
        observableWindow.setBackground(Color.black);
        gameWindow.setVisible(true);
        gameWindow.setSize(425,300);
        gameWindow.setDefaultCloseOperation(EXIT_ON_CLOSE);
        gameWindow.addKeyListener(new movement());
    }
    void startGame(){

        p1.setImage("/graphics/player.png");
        p1.setX(50);
        p1.setY(50);
        p1.paintPlayer();
    }
    void drawInvaders(int numInvaders){
        for(int i = 0; i < numInvaders; i++){
        int randX = (int) (Math.random() * this.getWidth());
        int randY = (int) (Math.random() * this.getHeight());
        invader a = new invader();
        a.setX(randX);
        a.setY(randY);
        a.newInvader();
        }
    }
}

这是错误代码

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at space.invaders.player.paintPlayer(player.java:31)
    at space.invaders.mainGame.startGame(mainGame.java:48)
    at space.invaders.mainGame$1.run(mainGame.java:26)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
BUILD SUCCESSFUL (total time: 12 seconds)